Aqueous dispersion of copolymer particles of vinyl acetate and a cyclic ketene acetal monomer

ABSTRACT

The present invention relates to a composition comprising an aqueous dispersion of copolymer particles comprising structural units of vinyl acetate, a cyclic ketene acetal monomer, and a monoethylenically unsaturated acid monomer, or a salt thereof; wherein the cyclic ketene acetal monomer is characterized by following structure: 
     
       
         
         
             
             
         
       
         
         
           
             where n and the R groups are as defined herein. The composition of the present invention provides a water-borne dispersion that is substantially free of volatile organic solvents and that provides a readily degradable polymer that is useful, for example, in the manufacture of biodegradable packaging products.

BACKGROUND OF THE INVENTION

The present invention relates to an aqueous dispersion of polymer particles comprising structural units of vinyl acetate and a cyclic ketene acetal monomer.

Copolymers of vinyl acetate (VA) and a cyclic ketene acetal (CKA) monomer such as 2-methylene-1,3-dioxepane (MDO) are particularly useful in applications that require polymer backbone degradation, such as degradable packaging, or the physical erosion of polymer-based thin films and surface coatings. These copolymers are described as being prepared in the presence of an organic solvent as disclosed, for example, in Polym. J. 2009, 41, 650-660; Polym. Chem. 2012, 3, 1260-1266; Polym. Chem. 2015, 6, 7447-7454; and US 1996/5541275.

Organic solvents are used to prepare CKA-containing polymers because CKAs are known to be hydrolytically unstable. For example, MDO hydrolyzes in water to form 4-hydroxybutyl acetate, thereby reducing the efficiency of incorporation of structural units of MDO into the copolymer backbone. Accordingly, the hydrolytic instability of MDO mandates the use of unreactive organic solvents; unfortunately, these solvents are undesirable due to their high cost, the cost associated with their removal, recovery, and recycling, and their contribution to remnant volatile organic solvents in the final isolated product.

Accordingly, it would be advantageous to find a way to prepare aqueous based VA-CKA copolymers in a way that significantly reduces the formation of unwanted hydrolytic byproducts.

SUMMARY OF THE INVENTION

The present invention addresses a need in the art by providing a composition comprising an aqueous dispersion of copolymer particles having a z-average particle size in the range of from 50 nm to 500 nm, which polymer particles comprise, based on the weight of the copolymer particles, a) from 75 to 98.5 weight percent structural units of vinyl acetate; b) from 1 to 20 weight percent structural units of a cyclic ketene acetal monomer; and c) from 0.05 to 5 weight percent structural units of a monoethylenically unsaturated acid monomer, or a salt thereof; wherein the cyclic ketene acetal monomer is characterized by following structure:

where n is 0, 1, or 2;

R is H or C₁-C6-alkyl;

R¹ and R² are each independently H, C₁-C₁₂-alkyl, phenyl, or vinyl; or R¹ and R² together with the carbon atoms to which they are attached, form a fused benzene ring or a fused C₃-C₇-cycloaliphatic ring; and

R¹′ and R²′ is are each independently H or C₁-C₁₂-alkyl; or R¹ and R¹′ and/or R² and R²′ form an exocyclic double bond;

with the proviso that when n is 1:

R³ and R³′ are each independently H, C₁-C₁₂-alkyl, phenyl, or R³ and R³′ form an exocyclic double bond or a spirocycloaliphatic group or spiro-2-methylene-1,3-dioxepane group;

with the further proviso that when n is 2:

each R³ is H, C₁-C₁₂-alkyl, or together with the carbon atoms to which they are attached form a double bond, a fused benzene ring, or a fused C₃-C₇-cycloaliphatic ring.

The composition of the present invention provides a water-borne dispersion that is substantially free of volatile organic solvents and that provides a readily degradable polymer that is useful, for example, in the manufacture of biodegradable packaging products.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a composition comprising an aqueous dispersion of copolymer particles having an z-average particle size by dynamic light scattering in the range of from 50 nm to 500 nm, which copolymer particles comprise, based on the weight of the polymer particles, a) from 75 to 98.5 weight percent structural units of vinyl acetate; b) from 1 to 20 weight percent structural units of a cyclic ketene acetal monomer; and c) from 0.05 to 5 weight percent structural units of a monoethylenically unsaturated acid monomer, or a salt thereof; wherein the cyclic ketene acetal monomer is characterized by following structure:

where n is 0, 1, or 2;

R is H or C₁-C₆-alkyl;

R¹ and R² are each independently H, C₁-C₁₂-alkyl, phenyl, or vinyl; or R¹ and R² together with the carbon atoms to which they are attached, form a fused benzene ring or a fused C₃-C₇-cycloaliphatic ring; and

R¹′ and R²′ are each independently H or C₁-C₁₂-alkyl; or R¹ and R¹′ and/or R² and R²′ form an exocyclic double bond;

with the proviso that when n is 1:

R³ and R³′ are each independently H, C₁-C₁₂-alkyl, phenyl, or R³ and R³′ form an exocyclic double bond or a spirocycloaliphatic group or a spiro-2-methylene-1,3-dioxepane group;

with the further proviso that when n is 2:

each R³ is H, C₁-C₁₂-alkyl, or together with the carbon atoms to which they are attached form an internal double bond, a fused benzene ring, or a fused C₃-C₇-cycloaliphatic ring.

As used herein, the term “structural unit of vinyl acetate” refers to a polymer backbone containing the following repeat units:

Structural Unit of Vinyl Acetate

wherein the dotted lines represent the points of attachment to the other structural units in the polymer backbone.

The term “structural unit of a cyclic ketene acetal monomer” is used to refer to a polymer backbone containing the following repeat unit:

Structural Unit of a Cyclic Ketene Acetal Monomer

where R, R¹, R², R³, R¹′, R²′, R³′, and n are as previously defined.

Examples of cyclic ketene acetal monomers include:

A preferred cyclic ketene acetal monomer is 2-methylene-1,3-dioxepane (MDO).

A structural unit of MDO is illustrated:

Structural Unit of 2-Methylene-1,3-Dioxepane

The ethylenically unsaturated acid monomer can be a carboxylic acid containing monomer, a phosphorus acid containing monomer, or a sulfur acid containing monomer, or salts thereof. Examples of suitable carboxylic acid containing monomers include acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid, and fumaric acid; examples of suitable phosphorus acid containing monomers include phosphoethyl methacrylate and 2-(methacryloyloxy)ethyl phosphonic acid; suitable sulfur acid containing monomers include 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), vinyl sulfonic acid, 2-sulfoethyl acrylate, 2-sulfoethyl methacrylate, 3-sulfopropyl acrylate, 3-sulfopropyl methacrylate, and 2-propene-l-sulfonic acid. The ethylenically unsaturated acid monomer is preferably a sulfonic acid containing monomer, preferably the salt of a sulfonic acid containing monomer, with a salt of 2-acrylamido-2-methylpropane sulfonic acid being especially preferred.

Preferably, the polymer particles have a z-average particle size in the range of from 80 nm, more preferably from 100 nm; to preferably 300 nm, more preferably to 200 nm, and most preferably to 150 nm.

Preferably, the polymer particles comprise, based on the weight of the polymer particles, from 80, more preferably from 85 weight percent; to 96.5, more preferably to 95 weight percent structural units of vinyl acetate; from 3 to 15, more preferably to 12, and most preferably to 8 weight percent structural units of the CKA, and preferably from 0.1 to 1 weight percent structural units of the monethylenically unsaturated acid monomer.

Moreover, the composition comprises a substantial absence of hydrolysis byproducts of the CKA and VA. More particularly, where the CKA is MDO, the composition preferably comprises less than 20, more preferably less than 10, more preferably less than 5, more preferably less than 2, and most preferably less than 1 weight percent of the undesirable ester 4-hydroxybutyl acetate (illustrated below) based on the weight of structural units of MDO in the polymer particles.

4-hydroxybutyl acetate

The composition preferably further comprises less than 3000 ppm, more preferably less than 2000 ppm of acetic acid, which is an undesired byproduct of VA hydrolysis.

The aqueous dispersion of the copolymer particles is advantageously prepared by contacting VA, the CKA, preferably MDO, and the monoethylenically unsaturated acid monomer, preferably a salt of a sulfur acid containing monomer, more preferably a salt of AMPS, under emulsion polymerization conditions. Preferably, the VA and the CKA are combined together, then mixed with an aqueous solution of surfactant and the monoethylenically unsaturated acid monomer under emulsion polymerization conditions. The reaction is advantageously controlled at a temperature in the range of from 30° C., preferably from 35° C., to less than 60° C., preferably to 55° C., more preferably to 50° C., and most preferably to 45° C., and at a pH in the range of from 6.0, preferably from 6.5, more preferably from 7.0, and most preferably from 7.5, to 9.0, more preferably to 8.5. After completion of the polymerization, a dispersion of the copolymer having a solids content preferably in the range of from 20, more preferably from 25, and most preferably from 30 weight percent, to 50, and more preferably to 40 weight percent, is obtained.

It has surprisingly been discovered that aqueous dispersions of copolymers of VA and CKA can be prepared efficiently without the use of organic solvents and with a minimum of undesirable hydrolysis byproducts of the CKA and VA.

Method for Measuring Particle Size

Particle size was measured using a Malvern Zetasizer Nano ZS90, which measures Z-average particle size (D_(c)) using dynamic light scattering (DLS) at a scattering angle of 90° using Zetasizer software version 7.11. A drop of the sample dispersion was diluted using an aqueous solution of MilliQ water (18.2 MΩcm at 25° C.) to achieve a particle count in the range of 200-400 thousand counts/s (Kcps). Particle size measurements were carried using instrument's particle sizing method and D_(z) was computed by the software. D_(z) is also known as the intensity-based harmonic mean average particle size and expressed as;

$D_{z} = \frac{\sum S_{i}}{\sum\left( \frac{S_{i}}{D_{i}} \right)}$

Here, S_(i) is scattered intensity from particle i with diameter D_(i). Detailed D_(z) calculations are described in ISO 22412:2017 (Particle size analysis—Dynamic light scattering (DLS)).

Acetic Acid Determination Method

Analysis of acetic acid was performed using a Agilent 1100 Series high-performance liquid chromatography system equipped with a Phenomenex Rezex ROA-Organic Acid H+(8%) (250×4.6) mm column (part number 00G-0138-E0), Phenomenex Security Guard Carbo-H4 element, UV detector operating at a wavelength of 210 nm, and autosampler. The column oven temperature was set to 35° C. and the mobile phase was 2.5 mM phosphoric acid in MilliQ water. The instrument operated at a flow rate of 0.4 mL/min (isocratic) and the sample injection volume was 5 μL. Data acquisition and analysis was performed using Agilent ChemStation software (version B.04.03). Samples were prepared for analysis by dilution in MilliQ water (1:100), followed by agitation on horizontal reciprocal shaker for 10 min. Samples were centrifuged at 100 000 rpm for 10 min at 25° C., and the supernatant was filtered through a 0.45 μm disposable syringe filter for injection.

EXAMPLE 1 Preparation of an Aqueous Dispersion of VA/MDO Polymer Particles

A monomer mixture was prepared in a vessel by combining vinyl acetate (VA, 205.95 g), and 2-methylene-1,3-dioxepane (MDO, 10.88 g). Separately, an aqueous mixture was prepared in a vessel by combining deionized water (46.18 g), 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt (AMPS salt, 1.40 g, 50% active in water), Tergitol-15-S-40 secondary alcohol ethoxylate (15-S-40, 1.51 g, 70% active in water) and Disponil® FES-32, sodium lauryl ether sulfonate (FES-32, 6.15 g, 31% active in water). Separately, deionized water (285.0 g) was added to a 4-neck 1-L round bottom reactor fitted with an overhead stirrer, a condenser, and thermocouple. The reactor was heated to 40° C., after which time FES-32 (16.4 g, 31% active in water), AMPS salt (3.95 g, 50% active in water), FeSO₄.7H₂O (16.2 g, 0.15 active in water), and ethylenediamine tetraacetic acid (EDTA, 3.45 g, 1.0 wt % in water) were added to the reactor with the aid of additional deionized water (51.6 g). The monomer mixture and the aqueous mixture were fed concurrently into the reactor over 60 min while maintaining the reactor temperature at 40° C. Concurrently, separately prepared solutions of ammonium persulfate/t-butyl hydroperoxide (1.38 g APS and 0.58 g t-BHP in 37.0 g water) and Bruggolite FF6 (2.62 g in 37.6 g water) were fed into the reactor over 70 min. The reaction was measured to be pH =6.5 to 7 throughout the feed process. Upon completion of addition of the feeds, the reactor temperature was maintained at 40° C. for 15 min, and then cooled to 30° C. The resultant dispersion was adjusted to pH =7 with the addition of ammonium hydroxide (28% active in water) and was filtered through a 38-μm screen. The filtrate was analyzed for percent solids (32.9%) and the z-average particle size was determined to be 116 nm, as measured using dynamic light scattering (DLS). The incorporation of 2-methylene-1,3-dioxepane (MDO) was measured to be (84.3±4.3)% by diffusion-edited ¹H NMR spectroscopy. An error of 5% was assumed in all integrations values and propagated through the calculation for the incorporation of MDO.

EXAMPLE 2

Example 1 was repeated, but the reaction was maintained at pH=8 throughout the feeds by the dropwise addition of ammonium hydroxide (28% active in water). The filtrate was analyzed for percent solids (31.3%) and the z-average particle size was determined to be 122 nm, as measured DLS. The incorporation of MDO was measured to be virtually quantitative (˜100%) by diffusion-edited ¹H NMR spectroscopy.

EXAMPLE 3

Example 2 was repeated, but the monomer mixture was prepared by combining VA (183.95 g), and MDO (32.88 g). The filtrate was analyzed for percent solids (29.7%) and the z-average particle size was determined to be 92 nm, as measured DLS. The incorporation of MDO was measured to be 97.8 ±6.6% by diffusion-edited ¹H NMR spectroscopy.

COMPARATIVE EXAMPLE 1

Example 1 was repeated, but the reaction temperature was maintained at 60° C. throughout the feeds. The filtrate was analyzed for percent solids (29.8%) and the z-average particle size was determined to be 177 nm, as measured DLS. The incorporation of MDO was measured to be 55.1±3.9% by diffusion-edited ¹H NMR spectroscopy.

COMPARATIVE EXAMPLE 2

Example 1 was repeated, but the reaction temperature was maintained at 80° C. throughout the feeds. The filtrate was analyzed for percent solids (29.4%) and the z-average particle size was determined to be 217 nm, as measured DLS. The incorporation of MDO was measured to be 37.8±2.7% by diffusion-edited ¹H NMR spectroscopy.

COMPARATIVE EXAMPLE 3

Example 1 was repeated, except that the reaction was maintained at pH=5 throughout the feeds by the dropwise addition of acetic acid (10% active in water). The filtrate was analyzed for percent solids (31.0%) and the z-average particle size was determined to be 248 nm, as measured DLS. The incorporation of MDO was measured to be 29.0±2.0% by diffusion-edited ¹H NMR spectroscopy.

COMPARATIVE EXAMPLE 4

Example 1 was repeated, but the reaction was maintained at pH=9.5 throughout the feeds by the dropwise addition of ammonium hydroxide (28% active in water). The filtrate was analyzed for percent solids (29.6%) and the z-average particle size was determined to be 283 nm, as measured DLS. The incorporation of MDO was measured to be 90.3 ±6.3% by diffusion-edited ¹H NMR spectroscopy; nevertheless, the sample displayed poor colloidal stability and 7850 ppm of gel was obtained. The sample discolored upon standing to a dark brown and an unusually high concentration of acetic acid and acetaldehyde was observed.

The examples demonstrate the significance of the role that temperature and pH play in optimizing the incorporation of the water-sensitive monomers into the polymer backbone and in reducing the production of unwanted hydrolysis products arising from these monomers. 

1. A composition comprising an aqueous dispersion of copolymer particles having a z-average particle size in the range of from 50 nm to 500 nm, which polymer particles comprise, based on the weight of the copolymer particles, a) from 75 to 98.5 weight percent structural units of vinyl acetate; b) from 1 to 20 weight percent structural units of a cyclic ketene acetal monomer; and c) from 0.05 to 5 weight percent structural units of a monoethylenically unsaturated acid monomer, or a salt thereof; wherein the cyclic ketene acetal monomer is characterized by following structure:

where n is 0, 1, or 2; R is H or C₁-C6-alkyl; R¹ and R² are each independently H, C₁-C₁₂-alkyl, phenyl, or vinyl; or R¹ and R² together with the carbon atoms to which they are attached, form a fused benzene ring or a fused C₃-C₇-cycloaliphatic ring; and R¹′ and R²′ are each independently H or C₁-C₁₂-alkyl; or R¹ and R¹′ and/or R² and R²′ form an exocyclic double bond; with the proviso that when n is 1: R³ and R³′ are each independently H, C₁-C₁₂-alkyl, phenyl, or R³ and R³′ form an exocyclic double bond or a spirocycloaliphatic group or spiro-2-methylene-1,3-dioxepane group; with the further proviso that when n is 2: each R³ is H, C₁-C₁₂-alkyl, or together with the carbon atoms to which they are attached form a double bond, a fused benzene ring, or a fused C₃-C₇-cycloaliphatic ring.
 2. The composition of claim 1 wherein the polymer particles comprise, based on the weight of the polymer particles, a) from 85 to 96.5 weight percent structural units of vinyl acetate; b) from 3 to 15 weight percent structural units of the cyclic ketene acetal monomer; and c) from 0.1 to 1 weight percent structural units of the monethylenically unsaturated acid monomer.
 3. The composition of claim 2 wherein cyclic ketene acetal monomer is selected from the group consisting of:


5. 4. The composition of claim 3 wherein the cyclic ketene acetal monomer is 2-methylene-1,3-dioxepane and the acid monomer is a sulfur acid containing monomer or a salt thereof.
 5. The composition of claim 4 wherein the sulfur acid containing monomer or salt thereof is selected from the group consisting of 2-acrylamido-2-methyl-1-propanesulfonic acid, salts of 2-acrylamido-2-methyl-l-propanesulfonic acid, vinyl sulfonic acid, salts of vinyl sulfonic acid, 2-sulfoethyl acrylate, 2-sulfoethyl methacrylate, 3-sulfopropyl acrylate, 3-sulfopropyl methacrylate, 2-propene-1-sulfonic acid and salts of 2-propene-1-sulfonic acid.
 6. The composition of claim 5 wherein the sulfur acid containing monomer or salt thereof is a salt of 2-acrylamido-2-methyl-1-propanesulfonic acid.
 7. The composition of claim 1 wherein the z-average particle size of the copolymer particles is in the range of from 80 nm to 300 nm.
 8. The composition of claim 5 which comprises less than 20 weight percent 4-hydroxybutyl acetate based on the weight of structural units of 2-methylene-1,3-dioxepane in the polymer particles.
 9. The composition of claim 8 which comprises less than 10 weight percent 4-hydroxybutyl acetate based on the weight of structural units of 2-methylene-1,3-dioxepane in the polymer particles.
 10. The composition of claim 7 which comprises less than 2 weight percent 4-hydroxybutyl acetate based on the weight of structural units of the cyclic ketene acetal monomer. 